In U.S. Pat. No. 4,381,934 there is disclosed a method for liquefying materials such as glass batch or the like, wherein a lining of pulverulent material such as the batch material itself acts as insulation to protect the vessel side walls from the intense temperatures within the vessel. In preferred embodiments, the lining encircles a central cavity of the vessel and the lining is maintained by permitting liquefied material to flow freely from the vessel and by feeding relatively cool material onto the lining. An advantage of such an arrangement is that high temperatures can be provided for liquefying the batch without requiring extensive contact of the product stream with contaminating refractory materials and without requiring extensive heat loss by forced cooling of the vessel walls.
Melting of a layer on a refractory wall has been known in the prior art, such as disclosed in U.S. Pat. Nos. 1,831,619; 1,889,511; 2,006,947; and 2,593,197. These arrangements have the drawback of placing a relatively small volume of throughput material in contact with a relatively large area of refractory material, thereby having a substantial potential for contaminating the throughput due to refractory erosion.
Prior art examples of water cooled melting vessels are disclosed in U.S. Pat. Nos. 2,834,157 (Bowes), 3,917,479 (Sayce et al.), and 4,061,487 (Kiyonaga). Each of these employs water cooling as the primary agent to preserve substantially the entire side wall portions of the vessel, with the result that a large amount of energy is wasted through the vessel walls due to the large temperature gradient established between the cooling fluid and the molten material within the vessel.
The liquefying method of the aforesaid U.S. Pat. No. 4,381,934 is capable of continuous operation over extended periods of time without the use of forced external cooling while avoiding significant thermal deterioration of the vessel side walls. Although the lining of batch material may at times erode irregularly, the system is generally self-repairing due to the feeding of additional batch material to the vessel. However, disruptions in the steady state conditions within the liquefaction vessel may occasionally result in reduction in thickness of portions of the lining, particularly in the upper portion of the vessel, to the extent that portions of the vessel side walls may be exposed to temperatures which, if sustained for a sufficient period of time, could lead to distortion or other thermal deterioration of the vessel. Instability of the batch wall thickness can occur, for example, during startup or when changing the batch feed rate or the heating rate. Also, during normal operation the batch lining at a middle elevation may erode more rapidly, thereby undercutting the lining above, and eventually causing a sudden collapse of lining from an upper region. Such aberrations do not present a significant problem if they are short in duration, but if they are sustained or occur frequently, it would be desirable to provide thermal protection for the vessel. Thermal distortion of the vessel is particularly a problem because of the dynamic instability it produces when the vessel is rotated in accordance with the preferred embodiments of the liquefaction process.
U.S. Pat. No. 3,689,679 (Niwa et al.) discloses a silica melting process of a somewhat different type in which portions of the vessel are cooled. As depicted in the patent, cooling is provided on a majority of the side wall area in the region of active melting. The patent does not involve liquefying at the surface of a lining as in the present invention.